Heretofore within said conventional tool holders there have been removably mounted an elongated cut-off blade of uniform thickness and which has along its top edge an enlarged head which is normally concave along its top surface throughout its length and whose side walls are tapered downwardly and inwardly. The leading edge of the blade including the head is cut downwardly and rearewardly to define with said head an irregularly shaped cutting edge. From time to time as this cutting edge wears, the blade itself is ground in planes parallel to its initial downwardly and rearwardly inclined edge to thus provide in said head a new cutting edge. This process can be repeated from time to time, regrinding the blade so as to provide new cutting edges.